Modulation and Detection
John Staples, LBNL November 2007
Linear Sum of Two Signals
Equation of sinusoidally-varying voltage: V t = V 0 cos 0 t 0
A linear sum of two waves:
V t = V 10 cos 1,0 t 1,0 V 20 cos 2,0 t 2,0
No new frequencies are generated.
Modulation is a non-linear process that generates additional frequencies.
Amplitude Modulation
This is a non-linear process, where the amplitude of the signal is modulated by another signal.
V t = V 0 1 m cos m t m cos c t c where 0 < m < 1 is the modulation index, um and uc are the carrier and the modulation frequencies. The first term in the parentheses represents the carrier, and the second term the modulation. Using the trig identity
1 cos A cos B = cos A−Bcos AB 2 and ignoring the phases
m V t = V 0 cosc t cosm−ct cosmc t [ 2 ]
Three frequencies result, the carrier, and two sidebands. The carrier amplitude is unchanged, but the sideband amplitude is directly proportional to the modulation index m.
Graphical Representation of an Amplitude Modulated Signal
The envelope of the carrier reproduces The spectrum consists of the carrier the modulating waveform. However, and two sidebands, each separated by u from the carrier frequency. the carrier itself is not changing amplitude. c m V t = V 0 cosc t cosm−ct cosmc t [ 2 ]
In the frame of the rotating carrier vector, the
two sidebands rotate at a frequency of +uc
and -uc. Their vector sum (red) represents the envelope of the signal.
What would the envelope look like if the carrier were eliminated? Carrier and one sideband? Can an AM signal be overmodulated? Is the modulation information still there? Phase Modulation
The instantaneous carrier phase changes with the modulating waveform.
V t = V 0 cosc t = V 0 cos c t m cosm t
m is the modulation index and um is the modulating frequency. We will use a trig identity to expand this expression:
cos AB = cos A cos B − sin A sin B
V t = cosc t cosm cosm t − sinc t sinm cosm t V 0
This expression is further expanded with Bessel functions:
∞ k cos z cos = J 0 z 2∑ −1 J 2k z cos2 k k=1 ∞ k sin z cos = 2 ∑ −1 J 2k1 z cos2 k1 k=0
Phase Modulation Spectrum
The PM waveform is a double infinite series, with the carrier in quadrature to the nearest sidebands:
∞ V t k = cosc t J 0 m 2∑ −1 J 2 k m cos2 k m t V 0 [ k =1 ] ∞ k sin c t 2∑ −1 J 2 k1 m cos2k 1m t [ k =0 ]
The first few terms:
V t = J 0 m cosc t V 0 2 J 1 m sin c t cos m t
2 J 2 m cosc t cos2 m t
−2 J 3m sin c t cos3m t
−2 J 4 m cosc t cos4 m t
2 J 5 m sinc t cos5m t
2 J 6 m cosc t cos6 m t ⋯
Phase Modulation Spectrum
Finally, we use another trig identity: 2 sin A cos B = sin AB sin A−B to find 2 cos A cos B = cos AB cos A−B V t = J 0 m cosc t V 0 J 1m [sinc m t sin c − m t ]
J 2 m [cosc 2 m t cosc − 2m t ]
J 3 m [sin c 3 m t sinc − 3 m t ]
J 4 m [cosc 4 m t cosc − 4 m t ]
J 5m [sinc 5m t sinc − 5m t ] ⋯ which shows that the sidebands separated by num have an amplitude proportional to Jn(m). Note that each set of sidebands alternates in phase with respect to the carrier Jn(m)
Note that a modulation index m = 2.405 that the carrier disappears. This is the first zero of J0(z). At high modulation index, the phase of the sideband reverses. m Graphical Representation of a Frequency Modulated Signal Frequency modulation is a special case of phase modulation, where the phase shift is given by f max = f modulation
The spectrum consists of sidebands, spaced
by um, with phase dependent on sideband, and amplitude given by the Bessel functions. The envelope is constant, and the frequency varies with the modulation. In the frame of the rotating carrier vector, the sidebands modulate the phase. Here, the carrier amplitude is not constant, but when all sidebands are included, the carrier is constant.
Can an FM/PM signal be over- modulated?
Demodulation of an Amplitude Modulated Signal
The envelope of an Amplitude Modulated signal contains the information.
Ways to recover information from an AM signal:
1. Rectify and filter out carrier
2. Synchronously detect (sample)
Diode detector rectifies the modulated signal and filters in with the RC integrator.
Envelope Demodulation of an Amplitude Modulated Signal
The voltage across the RC integrator approximately follows the carrier envelope of the signal.
The non-linearity of the diode and the time constant of the integrator may result in some distortion of the recovered modulation.
Envelope modulation does not distinguish envelope variations caused by amplitude modulation, or envelope variations caused by, for example, a frequency modulated signal passed through a filter.
Synchronous Demodulation of an Amplitude Modulated Signal
Synchronous detection multiplies the signal with a carrier of the same frequency as the carrier of the original signal.
This way, the phase of the reinserted carrier is significant.
m V t = V cos t cos − t cos t × cos t 0[ c 2 m c m c ] c
Filter out the components with frequency 2ut:
V t 1 m = D t = cos− [ cosm t cos−m t ] V 0 2 4
Synchronous Demodulation
1 m Dt = cos− cos t cos− t 2 4 [ m m ]
The recovered information depends on the phase of the reinserted carrier
1 m =0 Dt = cos t In phase: 2 2 m m Quadrature: =/2 Dt = cos −/ 2 −sin t sin t = 0 4 [ m m ]
If the reinserted carrier is in phase, the recovered modulation um is recovered, along with a d.c. level.
If the reinserted carrier is in quadrature phase, no signal is recovered.
Synchronous demodulation is insensitive to noise in quadrature to the reinserted carrier, and also allows two independent signals to be modulated onto one carrier frequency (e.g. color television chroma information).
Similar schemes are used for single and double sideband, suppressed-carrier communications system.
Logical Phase Detector Circuit
Logical diagram: A signal and its inverse are switched to the output at the rate u2+f.
The switching wave is call the Local Oscillator (LO).
If the switching is in phase with the signal, the average output level integrates to 2/p.
If the switching phase is advanced 90 degrees, the output integrates to zero.
What if the LO is at 180 degrees? Practical Phase Detector Circuit
In the ideal circuit, the local oscillator (LO) performs the switching function. The signal (RF) is chopped, resulting in the Intermediate Frequency (IF) output.
LO LO
IF IF In the actual circuit, diodes are substituted for the switch, and are RF either saturated in the RF forward bias, or open in the reverse bias case.
Actual diodes are not perfect switching devices. They have a current conduction threshold which is temperature dependent. Actual mixers are usually operated with a high LO input level, saturating the diodes, and the IF output may have bias offsets due to mismatch of the diode characteristics.